The high packaging density of present day electronic equipment has made necessary the use of multilayer printed circuit boards. The latter, in addition to the layers on the opposite external surfaces thereof, are comprised of additional subsurface layers, which normally take the form of copper planes or traces imbedded in the board insulating substrate material.
The manufacture of multilayer printed circuit boards dictates the requirement for proper registration of the layers. More specifically, plated-through holes in a multilayer printed circuit board are required to be either electrically connected to or insulated from the internal planes or traces, in accordance with the electronic design implemented within the board. Misregistration, such as the shifting of one or more of the internal layers relative to the plated-through holes, may result in circuit shorts, opens or current leakage because of low resistance paths in the insulating substrate material. Such misregistration of the internal layers can be caused by a variety of factors, such as the shifting of the printed wiring artwork or inaccuracies in the latter, and thermal expansion or shrinkage during the laminating phase of board manufacture.
Test coupons are presently incorporated in printed circuit boards by the electronics industry to evaluate the quality of the printed wiring itself and its registration. These test coupons are placed in an isolated area of the printed wiring board and are adapted to be cross-sectioned. That is, the board is cut through with a fine-toothed saw and the internal traces are examined visually, usually with the aid of magnifying means. A visual determination may be made, for example, as to whether or not the specified amount of insulating material is present between a given trace and a standard via hole. It must be emphasized that this procedure is both time consuming and costly. Moreover, since misregistration of subsurface layers may occur in a random manner in any given manufacturing lot, it may be necessary to perform this cross-sectioning procedure on each and every board and in more than one area of the board to insure proper operation of the equipment in which they are ultimately placed.
What is desired is a test coupon arrangement which will provide an indication of the subsurface layer registrations without cross-sectioning the board and the time consuming visual examination of the traces; and which because of its simplicity, may be easily and swiftly performed on each and every board, and which may provide quantitative information on the degree of misregistration to enable corrective measures to be undertaken in the board manufacturing process. The present invention fills such a need.